Effects of Co on Mechanical Properties and Precipitates in a Novel Secondary−Hardening Steel with Duplex Strengthening of M2C and β−NiAl

Synergistic strengthening of nano−scaled M2C and β−NiAl has become a new route to develop ultra−high secondary−hardening steel. At present, the effect of Co on the synergistic precipitation behavior of duplex phases of M2C and β−NiAl has been rarely reported. This paper revealed the effects of Co on the mechanical properties and duplex precipitates of M2C and β−NiAl in a novel 2.5 GPa ultra−high strength secondary−hardening steel. The tensile tests indicated that a 10% Co−alloy steel achieved a much stronger secondary−hardening effects compared to a Co−free steel during aging process, especially in the early−aging state. Needle−shaped M2C and spherical β−NiAl particles were observed in both Co−alloy and Co−free steels. However, the number density, and volume fraction of M2C were significantly enhanced in the 10% Co−alloy steel. The Mo contents in M2C carbide and α−Fe after aging treatment were both analyzed through experimental determination and thermodynamic calculation, and the results indicated that Co decreased the solubility of Mo in α−Fe, thus promoting the precipitation of Mo−rich carbides.


Introduction
Ultra−high strength steel has been widely used in key load−bearing components of major equipment in the aerospace field, such as aircraft landing gears, rocket engine casings, engine power shafts, transmission shafts, etc. [1][2][3].At present, ultra−high strength steel mainly includes medium carbon low alloy ultra−high strength steel [4], secondary hardening ultra−high strength steel [5,6], martensitic aging steel [7][8][9], ultra−high strength stainless steel [10], etc.Under the same strength level, secondary−hardening ultra−high strength steel exhibits the best combination of strength, toughness, and fatigue performance.
As a typical representative of secondary−hardening steel, many studies on the microstructures and mechanical properties of AerMet 100 have been reported.There have been many studies on the microstructure and properties of AerMet 100 steel.Through controlling lath martensite with high dislocation density massive nano−scaled M 2 C carbides [11][12][13] and film−shaped reverse austenite [14], the tensile strength of AerMet 100 achieved 1960 MPa, and the fracture toughness reached 115 MPa m 1/2 .
In recent years, utilizing the synergistic strengthening of M 2 C and NiAl to obtain higher strength has also been focused on by many scholars [15][16][17].Wang [18,19] et al. developed a novel secondary−hardening steel with a tensile strength of 2020 MPa and fracture toughness of 105 MPa•m 1/2 by controlling the nano−scaled duplex precipitates of M 2 C and β−NiAl.Compared to the AerMet 100, the novel steel obtained an equivalent strength and toughness, while the contents of Ni and Co decreased from 24% to 14%, indicating a significant reduction in alloy cost.Liu [20]  The precipitation behavior of second phases during the aging process has a significant impact on the mechanical properties of secondary−hardening ultra−high strength steel, while Co has an important impact on the precipiatates.Speich [22] et al. and Heo [23] et al. indicated that Co could inhibit the recovery of dislocations during the aging process, providing more nucleation positions for strengthening precipitates.Won [24] et al. also indicated that Co increased particle number density and volume fraction of M 2 C, thus increasing the degree of secondary hardening.Liu [25] et al. reported that Co enhaced α'−Cr precipitates and accelerated precipitation kinetics of the Mo−rich phase during early−stage aging.Wang [26] et al. indicated that Co addition enhanced the precipitation of fully coherent B2−Ni(Al, Fe) and Mo−rich clusters via depleting both residual Al and Mo in the matrix.Overall, the addition of Co in ferrite decreases the chemical potential of alloying elements due to magnetic interaction and increased the activity of C and Mo elements, thus increasing the driving force of M 2 C carbides, leading to a decrease in the solubility of Mo in ferrite and enhancement of the precipitation of M 2 C carbides.
The synergistic strengthening of carbides and intermetallic compounds has become a new route for further developing higher strength secondary−hardening steel.However, the effect of Co on the synergistic precipitation behavior of duplex phases of M 2 C and β−NiAl was rarely reported.
In the present study, a novel secondary−hardening steel with duplex strengthening of M 2 C and β−NiAl has been designed, obtaining an ultra−high strength of 2.5 GPa and elongation of 10.5%.Combing experimental observation and thermodynamic calculations, the effects of Co on synergistic precipitates of M 2 C and β−NiAl were characterized and analyzed in detail.

Materials
In the present study, a novel secondary−hardening steel with duplex strengthening of M 2 C and β−NiAl precipitations were designed.Its nominal chemical compositions are listed in Table 1.In order to investigate the effect of Co on mechanical properties and precipitation behavior, a similar steel just without Co addition was also prepared.These two experimental steels (10% Co−alloyed and Co−free) were melted using vacuum−induction and casting, following homogenization treatment at 1200 • C and forged into round bar with a diameter of 15 mm.Then, the as forged steel was solution−treated at 1060 • C for 1 h and immediately oil quenched.Then, it was transferred to a cryogenic bath and held at −73 • C for 2 h to promote the transformation of residual austenite to martensite.In order to clarify the mechanical properties of the two steels during the aging process, samples aged at 480 • C for various holding times, including 0 s (after cryogenic treatment), 10 min, 30 min, 1 h, 5 h, 30 h, and 100 h were prepared, and water quenching was chosen after aging treatment.The detailed temperature profiles of heat treatment are shown in Figure 1.Then, the samples were machined into standard specimens for tensile tests and Charpy impact tests.

Results and Discussion
Figure 2 illustrates the variation in tensile properties during the aging process of the two steels.Initially, both materials exhibited comparable strength and ductility before aging treatment.As depicted in Figure 2a, the 10% Co−alloyed steel exhibited a continual increase in tensile strength with extended aging time, achieving a peak tensile strength of 2508 MPa at 5 h.Subsequently, further increasing the aging time to 30 h and 100 h led to a decrease in tensile strength.However, for the Co−free steel, an obvious drop in tensile strength was noted after 30 min of aging, and then reached its peak strength at 5 h.Through the precipitation strengthening, a strength improvement greater than the cryogenic specimen was obtained during the aging process, thereby contributing to the secondary hardening effect.Notably, the aging peak strength of the Co−free steel was comparable to that of the cryogenic specimen, while Co significantly enhanced the secondary hardening effect.This change of strength in the early aging was consistent with the results of Liu [25] et al. Figure 2b shows the evolution of yield strength.It can be seen that the yield strength gradually increased with the extension of aging time.A significantly higher yield strength was obtained in Co−containing steel.In contrast, for the 10% Co steel, when the aging time was less than 1 h, the elongation and reduction of area were less than 5% and 10%, respectively, exhibiting brittle fracture.When the aging time reached 5 h or more, good ductility was achieved.The ductility of the Co−free steel was superior to that of the 10% Co steel.Delagnes [17] et al. and Liu [25] et al. also reported similar results.
Overall, through the duplex strengthening of M 2 C and β−NiAl, a novel secondary− hardening steel with good balance of strength and plasticity has been developed in the present study, with a tensile strength of 2.5 GPa, an elongation of 10.5%, and an area reduction near to 50%.
Samples of 10% Co−alloyed steel and Co−free steel after aging for 5 h were selected for precipitation phase analysis using 3DAP, and the results are shown in Figures 3 and 4, respectively.Through the elements distribution maps in Figure 3a, it was found that after aging for 5 h, C, Cr, Mo, Ni, and Al elements all exhibited significant segregation.The distribution of C, Cr, and Mo were relatively consistent, as the distribution of Ni and Al were also relatively consistent, indicating the formation of needle−shaped M 2 C and spherical β−NiAl precipitates.And the same phenomenon was also observed in Co−free steel, as shown in Figure 4a.
The composition distribution of individual β−NiAl and M 2 C precipitates are given in (b) and (c) in Figures 3 and 4  Based on the characterization results of 3D−APT, the volume fraction, number density, average volume, and nominal radius of precipitates in the two experimental steels were further statistically analyzed, as listed in Table 2.It can be seen that with the addition of 10% Co, the volume fraction of M 2 C significantly increased from 1.09% to 1.99%, and the number density also increased from 2.734 × 10 17 /cm 3 to 5.596 × 10 17 /cm 3    Because of the too small a size of the precipitate after aging for 5 h, samples after aging for 100 h were chosen to further be observed using TEM, as shown in Figure 5.The bright and dark field images indicated that a large number of needle−shaped precipitates existed in the matrix, which could be identified as M 2 C through fast Fourier transform (FFT) and element mappings.For the β−NiAl phase, it was hardly directly observed through bright and dark field images due to a small contrast to the matrix.The FFT image indicated that a series of weak patterns existed at 1/2 of the (200) α direction, which was β−NiAl and exhibited a cube−on−cube relationship with the matrix [27,28].The obvious element enrichment in Figure 5d also provided evidence of the presence of M 2 C and β−NiAl.From the comparison between Figure 5(a2,e2), it also can be found that the number density of M 2 C in 10% Co−alloyed steel was higher than that in the Co−free steel.The number density of M 2 C and β−NiAl corresponded to their nucleation number density, which was mainly related to the number of nucleation sites, including dislocations, grain boundaries, vacancies, and pre−existing precipitated particles.Previous study [22,23] has reported that Co can hinder dislocation recovery during the aging process, providing nucleation sites for precipitates.Due to an extremely low mismatch with the α−Fe, the nucleation mode of β−NiAl was mainly heterogeneous nucleation in the matrix [29].
However, the nucleation mode of M 2 C carbide both included nucleation in the matrix and on the β−NiAl particles.Therefore, the formation of β−NiAl can also promote the nucleation of M 2 C carbide, corresponding to a significant increase and a slight increase in number density of M 2 C and β−NiAl, respectively.
The effect of Co on volume fraction of M 2 C carbide was related to the solubility of Mo in the matrix [30].The Mo contents in M 2 C particles and matrix were analyzed using 3D−APT, and the results are shown in Figure 6a.It can be seen that after adding 10% Co, the Mo content in the matrix significantly decreased as that in the M 2 C carbide increased.The variation of Mo contents in the matrix were also calculated using Thermo−Calc software, and the Co content varied from 1% to 15%, as shown in Figure 6b.It can be seen that as the Co content increased, the Mo content in the matrix gradually decreased, which corresponded with the experimental results.The above research indicated that the addition of Co can reduce the solubility of Mo in the matrix, promoting much more Mo elements to form precipitates.Moreover, similar conclusions on the effect of Co on the solubility of Mo in maraging steel were also reported by Sha [31] et al.

Conclusions
The effects of Co on the mechanical properties and duplex precipitates of M 2 C and β−NiAl were investigated in a novel 2.5 GPa ultra−high strength secondary−hardening steel.The 10% Co−alloy steel after quenching and cryogenic treatment obtained a similar strength compared to the Co−free steel, but the peak strength of the former during the aging process was much higher than that of the latter, exhibiting much stronger secondary hardening effects with Co addition.The precipitates characteristics indicated that needle−shaped M 2 C and spherical β−NiAl were formed in both steels.However, the addition of Co significantly increased the number density and volume fraction of M 2 C, and slightly increased the number density of β−NiAl.Experimental determination and thermodynamic calculations indicated that Co can decrease the solubility of Mo in α−Fe, thus promoting the precipitation of Mo−rich carbides.

Figure 1 .
Figure 1.Temperature profiles of the experimental steels during heat treatment.
2.2.Experimental MethodsTensile tests were conducted at room temperature on an MTS−−880 universal material testing machine (MTS−880, MTS Corporation, Woodbury, MN, USA), with a sample size of Φ5 mm × 25 mm and a stretching rate of 1×10 −2 m/min.Atom probe tomography (APT) tests were performed using a local electrode atom probe (LEAP 4000X Si, CAMECA, Madison, WI, USA ), in which needle−shaped specimens for tests were prepared by a standard two−stage electropolishing procedure.Data reconstruction and analyses were conducted using IVAS 3.8.4.The volume fraction of austenite and dislocation density were measured using an X−ray diffractometer (XRD, Philips APD−−10, Philips Corporation, Amsterdam, The Netherlands).Thin transmission electron microscopy (TEM) foils were prepared from 3 mm diameter discs and ground to 50 µm thickness, then electro−polished.Detailed observation of microstructures and precipitates was conducted using TEM (Hitachi H−−900, Hitachi Corporation, Tokyo, Japan) with an acceleration voltage of 200 kV.Thermodynamic calculations were carried out using Thermo−Calc 2022 software (Thermo−Calc AB Corporation, Stockholm, Sweden) with the TCFE10 database.

Figure
Figure 2c,d show the elongation and area reduction of the experimental steels.It is evident that the ductility of the Co−free steel generally increased with increasing aging time.In contrast, for the 10% Co steel, when the aging time was less than 1 h, the elongation and reduction of area were less than 5% and 10%, respectively, exhibiting brittle fracture.When the aging time reached 5 h or more, good ductility was achieved.The ductility of the Co−free steel was superior to that of the 10% Co steel.Delagnes[17] et al. and Liu[25] et al. also reported similar results.Overall, through the duplex strengthening of M 2 C and β−NiAl, a novel secondary− hardening steel with good balance of strength and plasticity has been developed in the present study, with a tensile strength of 2.5 GPa, an elongation of 10.5%, and an area reduction near to 50%.Samples of 10% Co−alloyed steel and Co−free steel after aging for 5 h were selected for precipitation phase analysis using 3DAP, and the results are shown in Figures3 and 4, respectively.Through the elements distribution maps in Figure3a, it was found that after aging for 5 h, C, Cr, Mo, Ni, and Al elements all exhibited significant segregation.The distribution of C, Cr, and Mo were relatively consistent, as the distribution of Ni and Al were also relatively consistent, indicating the formation of needle−shaped M 2 C and spherical β−NiAl precipitates.And the same phenomenon was also observed in Co−free steel, as shown in Figure 4a.The composition distribution of individual β−NiAl and M 2 C precipitates are given in (b) and (c) in Figures 3 and 4, respectively.For β−NiAl, Ni and Al contents gradually increased from the edge to the center, while the C, Cr, and Mo contents reached maximum in the transitional region between β−NiAl and the matrix, then decreased in the core of β−NiAl.It may indicate that C, Cr, and Mo are expelled from the core with the nucleation and growth of β−NiAl.It can also be clearly observed that Ni and Al elements were enriched in the transition zone between M 2 C carbide and the matrix.
Figure 2c,d show the elongation and area reduction of the experimental steels.It is evident that the ductility of the Co−free steel generally increased with increasing aging time.In contrast, for the 10% Co steel, when the aging time was less than 1 h, the elongation and reduction of area were less than 5% and 10%, respectively, exhibiting brittle fracture.When the aging time reached 5 h or more, good ductility was achieved.The ductility of the Co−free steel was superior to that of the 10% Co steel.Delagnes[17] et al. and Liu[25] et al. also reported similar results.Overall, through the duplex strengthening of M 2 C and β−NiAl, a novel secondary− hardening steel with good balance of strength and plasticity has been developed in the present study, with a tensile strength of 2.5 GPa, an elongation of 10.5%, and an area reduction near to 50%.Samples of 10% Co−alloyed steel and Co−free steel after aging for 5 h were selected for precipitation phase analysis using 3DAP, and the results are shown in Figures3 and 4, respectively.Through the elements distribution maps in Figure3a, it was found that after aging for 5 h, C, Cr, Mo, Ni, and Al elements all exhibited significant segregation.The distribution of C, Cr, and Mo were relatively consistent, as the distribution of Ni and Al were also relatively consistent, indicating the formation of needle−shaped M 2 C and spherical β−NiAl precipitates.And the same phenomenon was also observed in Co−free steel, as shown in Figure 4a.The composition distribution of individual β−NiAl and M 2 C precipitates are given in (b) and (c) in Figures 3 and 4, respectively.For β−NiAl, Ni and Al contents gradually increased from the edge to the center, while the C, Cr, and Mo contents reached maximum in the transitional region between β−NiAl and the matrix, then decreased in the core of β−NiAl.It may indicate that C, Cr, and Mo are expelled from the core with the nucleation and growth of β−NiAl.It can also be clearly observed that Ni and Al elements were enriched in the transition zone between M 2 C carbide and the matrix.
. The volume fraction of β−NiAl remained basically unchanged and its number density slightly increased.Moreover, the average volumes of individual M 2 C and β−NiAl decreased.

Figure 5 .
Figure 5. TEM images of precipitates in the steel after aging for 100 h.(a-d) 10% Co−alloyed steel, (e) Co−free steel.(a 1 ,a 2 ) bright field and dark field images, (b 1 ) high−resolution TEM image, (b 2 ) fast Fourier transform (FFT) of M 2 C corresponding to yellow circle in (b 1 ), (c 1 ,) high−resolution TEM image, (c 2 ) FFT of β−NiAl corresponding to yellow square in (c 1 ), (d) element maps, and (e 1 -e 3 ) bright field and dark field images.

Figure 6 .
Figure 6.(a) Statistical results of Mo contents in matrix and M 2 C carbide through 3D−APT, (b) Calculation results of Mo contents in matrix after the aging process with different Co contents using Thermo−Calc software.
et al. also developed a novel 2.4 GPa extra−high strength steel with good ductility of 11.4% total elongation.Zhu [21] et al. also reported a novel cost−effective secondary−hardening steel with an ultimate tensile strength of 2185 MPa, due to the shear strengthening of NiAl and Orowan strengthening of M2C carbides.The excellent mechanical properties of the steel could be well interpreted by synergistic strengthening of high−density dislocations, massive nano−sized M 2 C, and NiAl precipitates.

Table 2 .
Statistical analysis of precipitates in the two experimental steels.